The maximizing of power output in microwave systems such as radar systems is a continuing objective. Two types of microwave amplifiers that have long been used as the output stage of high power microwave systems are the traveling wave tube and the cross field amplifier. Each have their particularly desirable characteristics and limitations. The traveling wave amplifier is a relatively broad band device operating over at least an octave and one which can simultaneously amplify a number of frequencies in the microwave range. Typically, traveling wave tubes are operated in S and X bands. Cross field amplifiers produce high power output but are relatively narrow band devices compared with traveling wave tubes.
Parallel operation of traveling wave tubes is commonly used to multiply the output power to a single port. Performance is however limited by the variation in phase shift between different traveling wave tubes.
The cold matrix structure of a traveling wave tube is long enough to accumulate many wave lengths at any particular frequency. For example, an X-band tube may contain some 40 wave lengths. Non-uniformities due to both periodic and aperiodic mismatches along the helix perturb this characteristic to add minor "cold" differences due to helix length. Thus, the variation in electrical length is in the order of .+-.10.degree. for controlled production tubes. Phase shifts caused by power supply voltage changes range from 0.5.degree. per volt to 2.5.degree. per volt. Phase shifts caused by changes in input power level are in the order of 4.degree. to 5.degree./db.
A second but similar application of traveling wave tubes is to drive individual elements of an antenna array. Adoption of traveling wave tubes for use in such phased arrays depends on inclusion of suitable auxiliary circuits to equalize the phase shifts from tube to tube. Typical circuits for such phase equalization employ the output of one traveling wave tube to phase control the next after a phase comparison. Numerous researchers have studied the problems of parallel high power traveling wave tube operation and have yet to arrive at a satisfactory solution.